1. Field of the Invention
The present invention relates to a polypropylene-based resin composition, which is excellent in moldability during injection molding, appearance of welded section, flexural modulus of elasticity and resistance to impact, and suitable for injection-molded parts for automobiles
2. Discussion of the Related Art
A number of attempts have been made in the past to improve impact resistance and rigidity of polypropylene resin by blending it with an ethylene/propylene polymer, various types of ethylene-based copolymer and talc. For example, resin compositions of excellent resistance to impact are disclosed by Japanese Patent Publication Nos. 63-42929 and 64-150, and Japanese Laid-open Patent Application Nos. 64-66263 and 1-204946.
More recently, resin compositions are required to be higher in fluidity and rigidity to satisfy the demands for shortened molding cycles, and thinner products and larger parts. The above compositions are insufficient in moldability for the above purposes.
An attempt has been made to solve the above problems, as disclosed by Japanese Laid-open Patent Application No. 7-53843. This composition, however, is still insufficient in fluidity to satisfy the requirements by a high-cycle molding process affected at low pressure, which needs still higher fluidity.
It is generally accepted that increasing the melt flow rate (MFR) of a resin composition to improve its moldability tends to be accompanied by deteriorated appearance of the welded sections of the molded product thereof. Excessive deterioration of the weld line lowers the product value.
Resin compositions free of the problems associated with weld appearance are proposed by, e.g., Japanese Laid-open Patent Application Nos. 9-263665 and 9-71714. They are good in weld appearance, but insufficient in fluidity.
It is an object of the present invention to provide a polypropylene-based resin composition, exhibiting high fluidity, while retaining good additional properties and weld appearance; excellent in moldability and molding fabricability; and suitable for automobile exterior parts, e.g., bumpers, while solving the above problems.
The inventors of the present invention have found that a polypropylene-based resin composition showing high fluidity, and good additional properties and weld appearance, and which is excellent in moldability and molding fabricability can be obtained by incorporating a propylene/ethylene block copolymer having a propylene homopolymer portion of high fluidity and very high crystallinity with a propylene homopolymer component of low molecular weight, ethylene/xcex1-olefin copolymer rubber and/or ethylene/xcex1-olefin/non-conjugated diene copolymer rubber, and/or hydrogenated block copolymer rubber, and talc having a specific range in particle size, reaching the present invention.
More concretely, the present invention provides the polypropylene-based resin composition, comprising the following components (A) to (D):
(A): 45 to 85 wt. % of a propylene/ethylene block copolymer, having a melt flow rate (MFR) of 100 g/10 min. or more for the propylene homopolymer portion and 50 to 200 g/10 min. for the total copolymer,
(B): 0.5 to 20 wt. % of a propylene homopolymer having a weight-average molecular weight (Mw) of 10,000 to 110,000,
(C): 5 to 35 wt. % of an ethylene/xcex1-olefin copolymer rubber and/or ethylene/xcex1-olefin/non-conjugate diene copolymer rubber, and/or hydrogenated block copolymer rubber, having an MFR of 0.1 to 20 g/10 min, and
(D): 5 to 30 wt. % of talc having an average particle size of 10 xcexcm or less, determined by the laser diffraction method, where the % weights are based on the total weight of the composition.